1. Field of the Invention
The present invention relates to an organic electroluminescence device, and more particularly, to an organic electroluminescence device capable of improving the uniformity of light emission and retarding the lowering in initial luminances.
2. Description of the Relates Arts
Since an electroluminescence device (hereinafter referred to as EL device) has features in a high self-distinguishability because of its self-emission, and having a high impact resistance because it is a completely solid device, various devices using inorganic and organic compounds are proposed at present and attempts to put them into practice use have been made. Among these devices, organic EL devices permit drastically low voltage to be applied, and therefore developments of various materials for these devices as well as devices have been undertaken.
The above organic EL device basically comprises anode/light emitting layer/cathode, and those which are provided with a hole injecting layer or electron-injecting layer at need for improving the light emitting property. In this structure of the device, the cathode must be sufficiently adhered to the light emitting layer. If the adhesivity is not sufficient, the mechanical strength of the device becomes low, causing ununiform light emission, and in the worst case non-emission regions. Furthermore, ununiformity of the load in the light emitting face is caused, which accelerates the deterioration to shorten the lifetime of the device, and this is an obstacle to practical use of the device.
Heretofore, it has been known that an electron-injecting layer or a hole barrierer layer is provided, at need, between the light emitting layer and the cathode. In these techniques, particularly when the latter layer is provided, the material is selected depending on the difference from the light emitting layer in the energy level, and the material must fit to this concept. An example of them is a technique of "providing a hole blocking layer (hole barrier layer) having a first oxidation voltage 0.1 V higher than that of the light emitting layer, between the light emitting layer and the cathode" (Japanese Patent Application Laid-Open No. 195683/1990). Also in Japanese Patent Application Nos. 195683/1990 and 255788/1990, 8-hydroxyquinoline derivative is used as the hole barrier layer basing on the above concept, but the emission efficiency of blue lights in these arts are still so low as 0.3 (1 m.W.sup.-1). On the other hand, when the materials described in Japanese Patent Application Laid-Open No. 231070/1991 and Japanese Patent Application No. 279304/1990 are used for the light emitting layer, an emission of blue light in a high brightness can be obtained. These materials are mentioned as effective materials for full-colorization of flat panel displays and the like in future. However, when these materials are formed to devices in the structure of anode/light emitting layer/cathode/, or anode/hole injecting layer/light emitting layer/cathode as described before, ununiform emission or non-emission region are sometimes caused, which have brought about problems in analyzing the practical use of the devices concerning lifetime of the device, minute processing of the device.
On the other hand, as the means to dissolve the above-mentioned problems, the doping technique (Japanese Patent Application Laid-Open No. 255190/1991, The Institute of Electronics, Information and Communication Engineers(IEICE) Technical Report (vol. 91, No. 406(1991), p47), Polymer Preprints Japan (vol. 40, No. 10 (1991))) and the like have been disclosed.
These aim at improving the properties by contaminating a second component into the light emitting layer. However, when the second component is contaminated into the light emitting layer, a possible fall in emission efficiency or change in the color of the emitting light have had to be considered sufficiently.